1. Field of the Invention
The present invention relates to a ranging channel processing apparatus of a communication system and a method thereof.
2. Description of the Related Art
Among communication systems, an Orthogonal Frequency Division Multiple Access (OFDMA) communication system is a multi-user access based system that allocate subchannels formed by a set of subcarriers to each User Equipment (UE) and thus the OFDMA-based system uses more subcarriers that a conventional OFDM system (e.g., IEEE 802.11a) does.
When a base station of the OFDMA system transmits signals to a plurality of mobile stations (i.e. during downlink), the signals are simultaneously transmitted through subchannels allocated to the respective mobile stations with time synchronization. Accordingly, orthogonality between signals can be guaranteed. However, when the plurality of the mobile stations transmit signals to the base station (i.e. during uplink), each of the plurality of mobile stations may have an individual time and location, and, therefore, signals transmitted from the plurality of mobile stations respectively arrive at the base station with different time delay and different transmitting power from each other. Particularly, a transmission signal of a mobile station that exceeds an allowable protection range of the base station causes orthogonality loss of a received signal, thereby resulting in a severe signal loss, and, therefore, the OFDMA system allocates a subcarrier group to a predefined ranging channel and the base station estimates a transmission delay and receiving power of each of the mobile stations by using ranging channels transmitted from the respective mobile stations.
For example, an Institute of Electrical and Electronics Engineers (IEEE) 802.16a/802.16e communication system defines four ranging modes. According to the ranging purpose, the ranging mode includes an initial ranging, a periodic ranging, a handover ranging, and a bandwidth-request ranging. The bandwidth-request ranging is used by a mobile station for requesting bandwidth allocation from a base station, and other ranging modes are used for uplink synchronization acquisition and power control between a mobile station and a base station.
A plurality of mobile stations are simultaneously allowed to transmit ranging channel signals, and each mobile station uses a ranging mode according to its purpose. For example, assume that “A” ranging codes are allocated for initial ranging, “B” ranging codes are allocated for periodic ranging, “C” ranging codes are allocated for handover ranging, and “D” ranging codes are allocated for bandwidth-request ranging. In this assumption, the ranging codes are respectively transmitted to the plurality of mobile stations through a DL_MAP message, and the mobile stations perform ranging by using appropriate ranging codes included in the DL_MAP message for the ranging mode.
A conventional transmitting apparatus of a mobile station and a conventional receiving apparatus of a base station, which use such a ranging method will be described.
Mobile stations trying to access the base station randomly select a ranging code among predetermined available ranging codes, modulate a signal by using a subcarrier group allocated to a ranging channel, transform a frequency domain signal into a time domain signal by using an Inverse Fast Fourier Transform (IFFT), and add a guard interval to the time domain signal so as to reduce inter-symbol interference. The mobile stations then transmit the time domain signal through a radio channel.
The base station receives signals having different time delays from the mobile stations, eliminates the guard interval added to the received signal, transforms the time domain signal into a frequency domain signal by using FFT, and correlates a ranging channel signal obtained by performing the FFT on the time domain signal with a base station ranging code, and estimates a time delay and receiving power.
Since the base station does not know which ranging mode is used to perform a ranging process by each of the mobile stations, the base station therefore arranges ranging codes used for the respective ranging modes in parallel for correlation corresponding to a ranging length (N) for each ranging code. However, such a correlation process must be repeated with respect to all time delays that can be searched, causing the size of hardware to increase and the amount of computations to increase. Accordingly, too much time is required to estimate transmission delay and receiving power of a mobile station increases, thereby decreasing efficiency.
The above information disclosed in this Background section is provided to enhance the understanding of the invention and contain information that was not necessarily known to a person of ordinary skill in the art.